The most likely cause for the motor to speed up and slow down is a dry or worn walking belt or walking board. Lift the walking belt and feel underneath for worn spots on the board. If there are no worn spots you can lubricate the belt and board with part number 134591 belt lubricant from Sears. It will have an instruction sheet to tell you how to apply it.
You could consider putting the console through a calibration mode. Without a lot of technical expertise, there are risks involved. I would not really recommend you try this as electrical shock and damage to the controller are possible. These instructions tell you how to adjust the minimum speed by adjusting a potentiometer on the motor controller board:
These consoles have a calibration mode programmed into them that can assist the technician when trouble shooting the treadmill. This mode also allows the technician to calibrate the speed and incline of the treadmill whenever a major electrical component is replaced.
To enter calibration mode, press and hold the Stop and Speed s buttons while inserting the safety key. There are five levels to this mode. To move from one level to the next, press the Stop button. To exit this mode, remove the safety key at any time. Each level is described below. Level 1- Console Configuration
This level displays how the console has been configured to operate. There is a series of diodes on the back of the console that can be clipped to determine the maximum speed, type of motor controller, front roller size and maximum incline. This level also allows each button on the console to be checked for function. As each button is pressed, its numerical value will be displayed in the right half of the Time window.
DISTANCE- Software code version
TIME (Left ½)- Minimum PWM percentage of the speed control signal 6, 12, 15, or 21
TIME (Right ½)- Button test values
CALORIES-Roller size. 1.6, 1.9, or 2.5 (if an optic switch is used for measuring speed, ‘OP’ is shown)
SPEED- Maximum speed of the treadmill.
Level 2- Speed and Incline calibration
This level is used to calibrate the speed and incline of the treadmill. To calibrate the incline, press either Incline button. The treadmill will increase to its maximum incline, then down to the minimum incline.
To calibrate speed, press and hold the Speed s button until the Distance window displays an 85% duty cycle speed control signal. Adjust the MAX SPD potentiometer on the motor controller until the Speed window shows the correct maximum speed. Press Speed t and allow the treadmill to come to a complete stop on its own. This level also performs an internal check of the EPROM and EEPROM on the consolers circuit board. If these chips are not functioning correctly, the console will not work.
DISTANCE- PWM percentage of the speed control signal. 0–85
TIME- Internal test results. EP2P = pass, EF2F = fail
CALORIES- Maximum Incline 10, or 12
SPEED- Current speed of the treadmill
Level 3- LED Test.
This level is used to check the LED's on the console. When this level is first entered, the LED's will begin lighting one at a time until all the LED's are lit. The LED's will then turn off in the same order. The Speed buttons can be pressed to change the speed of the lighting pattern. Incline toggles all of the LED's on and off. Incline restarts the lighting